// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_THREADING_THREAD_LOCAL_STORAGE_H_
#define BASE_THREADING_THREAD_LOCAL_STORAGE_H_

#include "base/atomicops.h"
#include "base/base_export.h"
#include "base/macros.h"
#include "build/build_config.h"

#if defined(OS_WIN)
#include <windows.h>
#elif defined(OS_POSIX)
#include <pthread.h>
#endif

namespace base {

namespace internal {

    // WARNING: You should *NOT* be using this class directly.
    // PlatformThreadLocalStorage is low-level abstraction to the OS's TLS
    // interface, you should instead be using ThreadLocalStorage::StaticSlot/Slot.
    class BASE_EXPORT PlatformThreadLocalStorage {
    public:
#if defined(OS_WIN)
        typedef unsigned long TLSKey;
        enum : unsigned { TLS_KEY_OUT_OF_INDEXES = TLS_OUT_OF_INDEXES };
#elif defined(OS_POSIX)
        typedef pthread_key_t TLSKey;
        // The following is a "reserved key" which is used in our generic Chromium
        // ThreadLocalStorage implementation.  We expect that an OS will not return
        // such a key, but if it is returned (i.e., the OS tries to allocate it) we
        // will just request another key.
        enum { TLS_KEY_OUT_OF_INDEXES = 0x7FFFFFFF };
#endif

        // The following methods need to be supported on each OS platform, so that
        // the Chromium ThreadLocalStore functionality can be constructed.
        // Chromium will use these methods to acquire a single OS slot, and then use
        // that to support a much larger number of Chromium slots (independent of the
        // OS restrictions).
        // The following returns true if it successfully is able to return an OS
        // key in |key|.
        static bool AllocTLS(TLSKey* key);
        // Note: FreeTLS() doesn't have to be called, it is fine with this leak, OS
        // might not reuse released slot, you might just reset the TLS value with
        // SetTLSValue().
        static void FreeTLS(TLSKey key);
        static void SetTLSValue(TLSKey key, void* value);
        static void* GetTLSValue(TLSKey key);

        // Each platform (OS implementation) is required to call this method on each
        // terminating thread when the thread is about to terminate.  This method
        // will then call all registered destructors for slots in Chromium
        // ThreadLocalStorage, until there are no slot values remaining as having
        // been set on this thread.
        // Destructors may end up being called multiple times on a terminating
        // thread, as other destructors may re-set slots that were previously
        // destroyed.
#if defined(OS_WIN)
        // Since Windows which doesn't support TLS destructor, the implementation
        // should use GetTLSValue() to retrieve the value of TLS slot.
        static void OnThreadExit();
#elif defined(OS_POSIX)
        // |Value| is the data stored in TLS slot, The implementation can't use
        // GetTLSValue() to retrieve the value of slot as it has already been reset
        // in Posix.
        static void OnThreadExit(void* value);
#endif
    };

} // namespace internal

// Wrapper for thread local storage.  This class doesn't do much except provide
// an API for portability.
class BASE_EXPORT ThreadLocalStorage {
public:
    // Prototype for the TLS destructor function, which can be optionally used to
    // cleanup thread local storage on thread exit.  'value' is the data that is
    // stored in thread local storage.
    typedef void (*TLSDestructorFunc)(void* value);

// StaticSlot uses its own struct initializer-list style static
// initialization, as base's LINKER_INITIALIZED requires a constructor and on
// some compilers (notably gcc 4.4) this still ends up needing runtime
// initialization.
#define TLS_INITIALIZER \
    {                   \
        0               \
    }

    // A key representing one value stored in TLS.
    // Initialize like
    //   ThreadLocalStorage::StaticSlot my_slot = TLS_INITIALIZER;
    // If you're not using a static variable, use the convenience class
    // ThreadLocalStorage::Slot (below) instead.
    struct BASE_EXPORT StaticSlot {
        // Set up the TLS slot.  Called by the constructor.
        // 'destructor' is a pointer to a function to perform per-thread cleanup of
        // this object.  If set to NULL, no cleanup is done for this TLS slot.
        void Initialize(TLSDestructorFunc destructor);

        // Free a previously allocated TLS 'slot'.
        // If a destructor was set for this slot, removes
        // the destructor so that remaining threads exiting
        // will not free data.
        void Free();

        // Get the thread-local value stored in slot 'slot'.
        // Values are guaranteed to initially be zero.
        void* Get() const;

        // Set the thread-local value stored in slot 'slot' to
        // value 'value'.
        void Set(void* value);

        bool initialized() const
        {
            return base::subtle::Acquire_Load(&initialized_) != 0;
        }

        // The internals of this struct should be considered private.
        base::subtle::Atomic32 initialized_;
        int slot_;
    };

    // A convenience wrapper around StaticSlot with a constructor. Can be used
    // as a member variable.
    class BASE_EXPORT Slot : public StaticSlot {
    public:
        // Calls StaticSlot::Initialize().
        explicit Slot(TLSDestructorFunc destructor = NULL);

    private:
        using StaticSlot::initialized_;
        using StaticSlot::slot_;

        DISALLOW_COPY_AND_ASSIGN(Slot);
    };

private:
    DISALLOW_COPY_AND_ASSIGN(ThreadLocalStorage);
};

} // namespace base

#endif // BASE_THREADING_THREAD_LOCAL_STORAGE_H_
